Differential protective system



Nov. 6, 1951 LLOYD DIFFERENTIAL PROTECTIVE SYSTEM Filed April 19, 194

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Randall El. Lloyd, by /Wm His Attorney.

Patented Nov. 6, 1951 DIFFERENTIAL PROTECTIVE SYSTEM Randall E. Lloyd, Schenectady, to General Electric Company,

New York N. Y., assignor a corporation of Application April 19, 1948, Serial No. 21,924

4 Claims.

This invention relates to differential protective systems and their application for protection against faults in electrical circuits.

An object of my invention is to provide an improved diiferential protection system for protection against faults.

Another object of my invention is to provide a differential protection device for electrical equipment which will not chatter or operate improperly under abnormal conditions of temperature, or extreme electrical conditions other than faults.

My invention, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

In the drawings, in which like parts are indicated by like reference numerals, Fig. 1 is a par-v tial view of one form of electromagnetic device which can be used in my differential protection system, Fig. 2 is a schematic diagram showing one application of my invention to direct current equipment, Figs. 3 and 4 are graphical representations of tripping characteristics of my differential protection system, and Fig. 5 is a diagram showing an application of my invention to the protection of alternating current equipment.

Referring to Fig. 1, the ends of the two sections l and I of the magnetizable core of the electromagnetic device or relay are separated by a small air gap 2 and have arcuate recesses which leave a circular opening 3 between the spaced apart core sections. In this space 3 a permanently magnetized armature 4 is pivotally mounted. A contact arm 5 carrying contacts 6 is mounted on the armature d. Stationary contacts l are located in a position to make electrical connection with the movable contacts 6 when the armature 4 and the contact arm 5 moves to one of its two operating positions.

Assuming that the armature has been magnetized so that the ends N and S of the armature A form the north and south poles of the magnet respectively, the armature will assume the position illustrated when the flux through the relay core is in such a direction that the tip of core section l adjacent the air gap is made the south pole and the tip of core section i is made the north pole. In this position, the contacts 6 and l are not engaged.

If the flux in the core is now reversed so that the tip of core section I becomes a south pole and 2 the tip of core section becomes a north pole, the pole N of the armature t will be attracted to the left, the pole S of the armature will be attracted to the right, and the armature will turn and close contacts 6 and I.

It should be noted that the relay described is directional. A reversal of the direction of magnetization of the magnetizable member of the relay and the consequent reversal of flux therein will cause the armature and the contacts to move from one of their two operating positions to the other. This is to be distinguished from the operation of the conventional relay in which a flux in either direction in the core will cause the armature and contacts to move to one of their two operating positions, while with no flux or very little flux the armature and contacts will move to their normal positions.

Referring now to Fig. 2, a direct current generator H feeds electric power to a load l2 through a distribution line l3. My differential protection circuit is shown as protecting against faults between the generator or the line I3 and ground. This is accomplished by the use of a conventional control circuit through which the contacts 6 and l of the relay 2| will open breakers M or deenergize the field [5 of the generator I l, or both, upon the occurrence of a fault. This control circuit can be of any conventional arrangement known or used in the art heretofore, and the particular form of the control circuits is not a part of my invention. However, I have illustrated diagrammatically such a control circuit for tripping the breakers Hi. When the contacts 6 and 7 of the relay 2! move to the closed position a current will flow from the battery [5, through the contacts 6 and l and through breaker tripping coils l8 of the breakers [4.

The relay 2! is shown located at the load end of the line 53. As actually shown, the generator current passes directly through the first winding 22 of the relay 25. However, if desired, a shunt could be used in the line l3 and the IR drop across the shunt used to excite a coil of a larger number of turns than the winding 22 shown. The field generated in the relay core by the coil 22 when the current is flowing in the normal direction from left to right will attract the armature 4 to a position in which contacts 6 and "i will maintain the operation of the load circuit, hereafter referred to as the power-on position of the contacts.

A second winding 23 is arranged to be energized by the IR drop across a shunt I! in the load circuit. If desired, the coil 23 can beenergized by the IR drop across the series field of the generator, but it has been found that the use of a shunt H of proper material will result in better temperature characteristics and is, therefore, more satisfactory where variable temperature conditions prevail.

When current is flowing through the generator circuit in the normal direction, the IR drop in the shunt l1 causes a current to flow from ground through winding 23, through resistor 24 and through adjustable resistor 25. This current is proportional to the IR drop in shunt l1 and, therefore, proportional to the load current through the generator. The field generated by the coil 23 tends to attract the armature 4 and the contacts 6 and I to the position in'which the contacts 6 and 1 will interrupt the current how in the generator circuit by opening breakers M or deenergizing the generator field [6, or both. This position of the relay contacts is hereafter referred to as the tripping position.

Under normal conditions, that is, when there is no fault between the generator H or the line l3 and ground, the field generated by the coil 22 in the relay core is slightly greater than the field generated by coil 23, and therefore the contacts 6 and! will be securely held in the power-on position. If a fault now occurs between the generator H and ground, or'between line 13 and ground, it is apparent that the field generated by the coil 22 will be substantially reduced while the field generated by the coil 23 will probably increase. This will result in the tripping of the relay 2! due to the reversal of the resultant of the magnetomotive forces of windings 22 and 23 and of the sense in which this resultant acts on the armature 4, thereby preventing serious damage which might otherwise be caused by the fault.

In certain applications in which the generator H is tied to a power network a surge of current may pass through the line [3 in the reverse direction. In such an event, the current through both windings 22 and 23 will be reversed and the fields generated by the two coils will be in reverse of their normal directions. Since the coil 22 has a greater effect on the armature 4 of the relay 2| than the coil 23, the reverse surge of current, if sufiiciently large, would cause the tripping of the relay 2! and the interruption of the load circuit. The interruption of the load circuit under these conditions is normally undesirable, since the reverse current surge would usually be temporary.

My solution of this difiiculty can best be understood if the operation of the relay is first shown graphically. In Fig. 3, I have shown the tripping characteristic of the relay on a graph in which load current is plotted against fault current. The tripping characteristic shows the fault current which will trip the relay at any given load current through coil 22. Ihe line A shows the tripping characteristic with the windings 22 and 23 arranged to generate equal fields under normal conditions. With the fields 22 and 23 adjusted in this manner, a reverse surge of current in the line 13 would not cause the interruption of the load circuit by the relay 2 i. However, this exact balancing of the fields generated by the windmgs 22 and 23 is not practical. Irregular tem perature variations and manufacturing tolerances would result in the variation of the pro posed relay characteristic A to characteristics indicated by the lines A and A". This would result in improper interruption of the load circuit were no fault. It is for this reason that the coil 22 is made to generate a substantially stronger field than the coil 23 under normal conditions. By substantially stronger I mean sufiiciently stronger that under any anticipated temperature conditions the relay will be held in the power-on position as long as there is no fault and the load current flows in the normal direction.

The resultant relay characteristic is shown by the line B in Fig. l. It will be noted that an increase in load current in the normal direction will not result in a tripping of the relay but will hold the relay in its power-on position more securely. However, a reverse surge of current through the load circuit will cause the relay to trip if the surge reaches a value designated in Fig. 4 as X since the characteristic line crosses the zero short circuit current axis at that point. As previously indicated, this tripping is not desirable since the reverse surge would normally be temporary and other means would be provided to correct the situation if the reverse now should be of substantial duration.

To avoid this improper tripping of the relay I have, in effect, changed the characteristic line of the relay by inserting in the differential protection circuit a rectifier 26. When current is passing through the load circuit in the normal direction, from left to right, current passes through the protection circuit from right to left and must pass through a portion of the adjustable resistor 25. The calibration of the relay is such that the characteristic line B is obtained and the relay operates properly when the load current is flowing in its normal direction.

When a reverse surge of current flows through the load circuit from right to left, the current in the protective circuit will flow from left to right. Since the rectifier 23 will allow current to pass from left to right, it is apparent that the adjustable resistor 25 will be by-passed. This results in a reduction of effective resistance in the protection circuit. The current through coil 23 will then be greater than if the adjustable resistor 2 5 were still effectively in the circuit.

Since, under reverse flow conditions, the flux generated by coil 23 attracts the relay armature 4 toward its power-on position, it will readily be understood that the increased effectiveness of the coil 23 will tend to prevent the undesired tripping of the relay by a reverse surge of current in the load circuit. Preferably, the amount of resistance by-passed by the rectifier is such that the characteristic line of the relay is change to that represented by the line B--B' in Fig. 4. With this characteristic line the relay will not trip under any condition in the load circuit other than a fault between the generator 1 L or the line 13 and ground.

In Fig. 2 I have shown the load current passing through the relay coil 22. In such a case it is necessary that the rectifier 26 be located in the electrical connections for relay coil 23. If, as suggested in the alternative above, a shunt were used in the line it and the IR drop across the shunt used to excite the coil 22, then the rectifier 26 could be located in the electrical connection for the coil 22. In that case the rectifier would be arranged to by-pass a resistor in the connection when the current in the line I3 flows in the normal direction. Then, in the event of a reverse surge of current through line iii, the current passing through coil 22 would also pass through the resistor with the result that coil 22, which under extreme load conditions even though in??? I? WQQ d then be the tripping coil, would be less effective than coil 23'which would then be the holding coil. This arrangement would then give the desirable tripping characteristic BB, shown in Fig. 4.

It should be noted that when my difierential protection circuit is used to protect a,generator or other power source the pick-up for the coil which is normally the tripping coil, coil 23 in Fig. 2, must be located in the grounded lead of the generator as illustrated in Fig. 2. On the other hand, when the circuit is used to protect a load device the pick-up for what is normally the holding coil, coil 22 in Fig. 2, must be located in the grounded lead of the load device. When the circuit is used to protect a transmitting conductor the pick-up for what is normally the tripping coil must be located at the end of the protected equipment which is connected to the normal power source. In general then, in any application of my circuit the pick-up for what is normally the holding coil must be in a lead which will be bypassed in the event of a fault to ground in the protected equipment and the pick-up for what is normally the tripping coil must be in a lead which will not be by-passed in the event of a fault to ground in the protected equipment.

When my differential protection circuit is ap plied to an alternating current circuit the characteristic line B-B' can be obtained without the use of a by-passing rectifier. This will be understood readily after my explanation of Fig. 5 which shows such an application. In Fig. 5 an alternating current generator 53! is shown feeding three phase power to a motor or other power consuming equipment through lines 33, 33 and 33". In the interest of clarity I have shown my differential protection circuit applied to only one of the three lines. Normally all three lines would be protected. Similarly, circuit breakers id have been shown in only one of the lines.

If a fault occurs in the line 33 the contacts 6 and l of the relay 2i will move to their tripping position and cause breakers i i to open or deenergize the generator field 36 or both. This control circuit can be of any conventional arrangement and the particular form is not a part of my invention. However, I have illustrated diagrammatically such a control circuit for tripping the breakers When the contacts 6 and l of the relay 2i move to the closed position a current will flow from the battery i=5, through the contacts s and l and through breaker tripping coils it of the breakers i l.

The current passing through the load end of the line 33 energizes the current transformer 3? which in turn causes an alternating current to flow to rectifier bridge 33. From the rectifier bridge a direct current flows through resistor 3.9 to the relay coil 22. The current passing through the coil 22 is proportional in magnitude to the alternating current passing through the load end of the line 33. Similarly, the current passing through the power source end of the line 33 energizes current transformer til which in turn causes an alternating current to flow to the rec tifier bridge 4i. From the rectifier bridge ii a direct current flows through resistor 32 to coil 23 of the relay 21. The current passing through the coil 23 is proportional in magnitude to the alternating current passing through the power source end of line 33.

The field generated by the coil 22 attracts the armature 4 and the switch contacts 5 and I to the power-on position. The field generated by the coil .23attracts the armature A and'the switch contacts 6 and l tothetripping position. Under normal conditions, that is, when there is no fault in the line 33, the field generated by the coil 22 is slightly greater as previously defined than the field generated by the coil 23. Therefore, under normal conditions the relay contacts 5 and l are securely held in the power-on position.

If a fault occurs in the line 33 the current passing through the power source end of the line 33 will be increased while the current passing through the load end of the line 33 will be decreased. The field generated by the coil 23 will then be stronger than the field generated by the coil 22 and the relay contacts (5 and 1 will move to their tripping position, thereby isolating or de-energizing the line 33 and preventing possible damage which would otherwise result from the fault.

In the event of a reverse surge of power through the line 33 the direction of flow of current in the coils 22 and 23 of the relay 2! will be unchanged since the rectifier bridges 38 and 4! will pass current in only one direction regardless of the direction of flow of power in the line 33. The tripplng characteristic BE' is therefore obtained and the relay contacts 6 and l will be securely held in their power-on position even though the power flows in the reverse direction in the line 33.

It should be noted that when my differential protection circuit is used to protect an alternating current generator or source of alternating current power the pick-up for the tripping coil, coil 23 in Fig. 5, must be located in the grounded lead of the generator. On the other hand, when the circuit is used to protect an alternating current load device the pick-up for the holding coil,

coil 22 in Fig. 5, must be located in the grounded lead of the load device. When the circuit is used to protect a power transmitting device for alternating current, such as a transmission line, a transformer, a synchronous condenser or a frequency changer, the pick-up for the tripping coil must be located at the end of the protected apparatus which is connected to the power source. In general then, in any application of my circuit to alternating current apparatus the pick-up for the holding coil must be located in a lead which will be by-passed in the event of a fault to ground in the protected apparatus and the pick-up for the tripping coil must be located in a lead which will not be by-passed in the event of a fault to ground in the protected apparatus.

This specification shows two specific applications of my differential protection circuit. It will be apparent to those skilled in the art that many other applications and many modifications and variations of the circuit are possible. It is m aim to cover all such modifications and variations in the circuit and its applications as fall within the scope of my invention as defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A protective system for a direct current circuit comprising, an electromagnetic device including a magnetized member and a magnetizable member coacting with said magnetized memher, one of said members being movably positioned in one of two operating positions dependent upon the direction of magnetization of said magnetizable member, a pair of excitation windings positioned in flux opposition on said magnetizable member, circuit means for energizing one of said windings approximately proportionany with the current in said direct current circuit at one extreme of the portion thereof to be protected, circuit means for energizing the other of said windings approximately proportionally with the current in said direct current circuit at the other extreme of the portion to be protected, said one winding normally predominating over said other winding whereby said magnetizable member is magnetized in a direction which maintains said one member in a first one of said two operating positions, means for changing the impedance of the energization circuit of one of said windings upon the reversal of current in said direct current circuit whereby said other winding predominates over said one winding and said direction of magnetization and the position of said movable member remain the same, the occurrence of a fault on said direct current circuit intermediate said extremes diverting at least a portion of the current from said first extreme and allowing said other winding to predominate whereby the direction of magnetization of said magnetizable member is reversed and said movable member is moved to the second of said operating positions, and means operative at said second position for deenergizing said direct current circuit.

2. A protective system for a direct current circuit comprising, an electromagnetic device including a magnetized member and a magnetizable member coacting with said magnetized memher, one of said members being movably positioned in one of two operating positions dependent upon the direction of magnetization of said magnetizable member, a pair of excitation windings positioned in flux opposition on said magnetizable member, circuit means for energizing one of said windings approximately proportionally with the current in said direct current circuit at one extreme of the portion thereof to be protected, circuit means for energizing the other of said windings approximately proportionally with the current in said direct current circuit at the other extreme of the portion to be protected, said one Winding normally predominating over said other winding whereby said magnetizable member is magnetized in a direction which maintains said movable member in a first one of said two operating positions, an impedance in the energization circuit of one of said windings, asymmetrical conducting means in shunt with at least a portion of said impedance to provide a different efiective value of impedance in said energization circuit for one direction of current flow than for the other direction whereby said other winding predominates over said one winding upon the reversal of current in said direct current circuit and the direction of magnetization of said magnetizable member and the position of said movable member remain unchanged, the occurrence of a fault on said direct current circuit intermediate said extremes diverting at least a portion of the current from said first extreme and allowing said other winding to predominate whereby the direction of magnetization of said magnetizable member is reversed and said movable member is moved to the second of said operating positions, and means operative at said second position for deenergizing said direct current circuit.

3. A difierential protective system for a direct current circuit comprising, an electromagnetic device including a permanently magnetized member and a magnetizable member coacting with said permanently magnetized member, said permanently magnetized member'being movably positioned in one of two Operating positions dependent upon the direction of magnetization of said magnetizable member, a pair of excitation windings positioned in flux opposition on said magnetizable member, circuit means for energizing one of said windings approximately proportionally with the current in said direct current circuit at one extreme of the portion thereof to be protected, circuit means for energizing the other of said windings approximately proportionally with the current in said direct current circuit at the other extreme of the portion to be protected, said one winding normally predominating over said other winding whereby said magnetizable member is magnetized in a direction which maintains said permanently magnetized member in a first one of said twooperating positions, an impedance in the energization circuit of said one winding, a rectifier in shunt with at least a portion of said impedance and poled to shunt said impedance upon reversal of current in said excitation circuit whereby upon the reversal of current in said direct current circuit said other winding predominates over said one winding and said direction of magnetization and the position of said permanently magnetized member remain the same, the occurrence of a fault on said direct current circuit intermediate said extremes diverting at least a portion of the current from said first extreme and allowing said other windin to predominate whereby the-direction of magnetization of said magnetizable member is reversed and said permanently magnetized member is moved to the second of said operating positions, and means operative at said second position for deenergizing said direct current circuit.

4. A differential protective system for a direct current circuit having a direct current generator therein at one extreme of the portion to be protected comprising, an electromagnetic device including a permanently magnetized member and a magnetizable member coacting with said permanently magnetized member, said permanently magnetized member being movably positioned in one of two operating positions dependent upon the direction of magnetization of said magnetizable member, a pair of excitation windings positioned in flux opposition on said magnetizable member, one of said windings bein connected in said direct current circuit at one extreme of the portion thereof to be protected, means for energizing the other one of said windings approximately proportionally with the current in said direct current generator at the other extreme of the portion of said direct current circuit to be protected, said one winding normally predominating over said other winding whereby said magnetizable member is magnetized in a direction which maintains said permanently magnetized member in a first one of said two operating positions, a resistor connected in circuit with said other winding, a rectifier connected in shunt with at least a portion of said resistor and poled to carry current upon the reversal of the current in said other Winding whereby said other winding predominates over said one winding and said direction of magnetization and the position of said permanently magnetized member remain the same upon a reversal of current in said direct current circuit, the occurrence of a fault on said direct current circuit intermediate said extremes shunting at least a portion of the current from said first extreme and allowing said other winding to predominate whereby the direction of magnetization of said magnetizable member is reversed and said permanently magnetized memher is moved to the second of said operating positions, and means operative at said second position for deenergizing said direct current generator and said direct current circuit.

RANDALL E. LLOYD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Sonnemann May 16, 1950 

